Process for removal of arsenic from lead containing it



Patented Nov. 30, 1943 PROCESS FOR REMOVAL OF ARSENIC FROM LEAD CONTAINING IT Max F. W. Heberlein, Rahway, and Nevin R. Bierly, 'Avenel, N. .L, assignors to The American Metal Company, Limited, New York, N. Y., a corporation of New York No Drawing. Application March 5, 1942, Serial No. 433,464

Claims.

This inventionrrelates to the removal of arsenic from impure lead, containing antimony and arsenic.

An important object of the invention is to provide a simple, eflicient process for removing arsenic from lead containing antimony without removing appreciable quantities of lead or antimony with the arsenic and to provide an arseniccontaining slag having a low content of other metals.

Other objects will be in part obvious and in part pointed out hereinafter.

In accordance with our invention it is possible to remove arsenic with no substantial amount of antimony and only a small amount of lead. This is a distinct advantage over the older processes where antimony and arsenic are removedtogether and must be separated subsequently by tedious and expensive methods.

Although our process may be operated in the presence of tin, it is preferred not to have tin present. If present, it appears in the dross with the arsenic and must be separated by subsequent treatment.

In working our process accordingly, we prefer to use lead from which the tin has already been removed. Any process capable of a selective tin removal may, of course, be used. We prefer the Harper and Reinberg process as described in Patent No. 2,155,545 which is highly efilcient for tin removal, bringing the tin content of the lead down to several hundredths of one per cent or traces of tin.

We have found in accordance with our invention that arsenic in lead containing arsenic and antimony can be converted into an oxidized compound, probably sodium arsenite, by sodium hydroxide, NaOH, without the aid of any other agent, while antimony will only react with sodium hydroxide after it has been oxidized by some oxidizing agent such as air, nitre, etc. Accordingly, a clean-cut separation of arsenic can be accomplished if a combination of arsenic, lead and antimony is treated with sodium hydroxide in the absence of air, whereby the arsenic is converted into a compound, probably arsenite, and can be removed by skimming.

A practical method of excluding air is to equip the well-known refining kettle, such as shown in the said Harper and Reinberg patent, with a tight-fitting cover. Experiments conducted with this typ of equipment have shown that arsenic can be completely removed in the manner indicated from lead on a commercial scale without appreciably afiecting the antimony or lead.

Arsenic contents of less than 0.01% in the treated metal have been obtained easily and the corre-- of solid sodium hydroxide is placed on the molten lead bath. The stirrer is put into position in the kettle, after which the air-tight cover is fastened to the kettle and the stirrer shaft. The cover may consist of a simple steel plate bolted or clamped to the kettle with an asbestos gasket between the cover and rim of the kettle. The stirrer shaft passes through a close-fitting opening in the center of the cover. Various modiflcations in the type of apparatus to insure the exclusion of air may of course be used. After the cover is firmly secured, the stirrer is started-and the caustic soda thoroughly stirred into the metal bath to insure intimate contact between the caustic and all parts of the metal bath. We believe the arsenic combines with the caustic to form a dross of NaiAsOa, according to the equation 3NaOH+As NaaAsOa+3H (dross) The dress is removed by skimming and will contain little lead and substantially no antimony. During the treatment, hydrogen is liberated as above indicated and burned at a vent, such as a bleeder valve, provided in the kettle. As the hydrogen flame diminishes, it indicates that the elimination of arsenic approaches its completion.

Some of the arsenic dross when leached and the liquors analyzed for trivalent and pentavalent arsenic have shown that the arsenic occurs in this solution in varying percentages (30-60%) in the trivalent form, but it is believed that during the kettle treatment under exclusion of air only NazAsO; is formed but immediately upon removing the cover from the treatment kettle, this compounds starts to oxidize and continues during the skimming period to form NasAs04. Oxidation at this time is very likely because the dross is handled and exposed to the air, while it has a temperature of above 650-700" F. This elevated temperature accelerates any oxidation and naturally would also promote the conversion ct trivalent arsenic into the pentavalent form.

We have found in accordance with our invention that the temperature to which the metal is heated has a direct bearing on the time required to remove the arsenic, all other conditions being the same. Thus, although we have operated our process below 000' F.,-we prefer to operate between 1000 and 1100' E, because the speed of removal is so much higher. a As stated above, a layer of sodium hydroxide is placed on the molten metal at the proper temperature using approximately 2 pounds for each pound of arsenic in the metal, which represents an excess of about 50% on the basis of NasAsO: formation.

An ordinary stirrer operating around 300 R. P. as. maybe used to agitate the bath or a highspeed impeller operating from 900 to 1700 R. P. M. as used in the Harper and Reinbera Patent No. 2,1555% may be used. We preier the high speed stirrer because it creates a more efllcientg and thorough mixing of the reacting substanwuq.1-

Other means of agitation may be employed if desired.

The time required for stirring depends on the temperature. ime of stirring apparatus. amount of arsenic present and other indicated variables, but in our preferred practice with the usual grades of lead containing around 59% of arsenic, 1 to 2 hours stirring is usually sufllcient to bring the arsenic content down to several hundredths of 1%. It may sometimes be desirable to remove only part of the arsenic, and in that event less reagent may be used and the stirring time correspondingly reduced. In order to determine whether the desired amount of arsenic has been removed, the metal is sampled and assayed. After operating the process for a short time, experience will soon teach the amount of reagent and the time required with a given set of conditions, in order to obtain the removal of arsenic desired.

After stirring for the desired length of time,

the slag is skimmed or! and leached with hot water aiter which it is filtered. The relatively small amount of residue or filter cake which contains some lead and. antimony may be returned to the lead smelter for the recovery of values contained. The filtrate containing the arsenic substantially free from other metals may be discarded, or may be converted to some form suitable for sale. Arensic slags produced by previous processes have been contaminated by other metals requiring further treatment to obtain the other metals or the arsenic or both.

The method which we have preferably used in order to exclude the air from the reaction has been the a plication of a simple tight-fitting cover as described above. However, in addition to a cover, an atmosphere of a neutral gas in the chamber between the top of the lead bath and the tizht-fitting cover may be provided, if desired. In this case an inlet, as well as an outlet pipe must be connected to the cover. so that the neutral gas can be fed in at the inlet and, by having the outlet Just slightly open, the gas can be kept under a slight pressure at all times. The introduction of a neutral gas. particularly at the end of the operation, will avoid sucking in of air under the cover and will eliminate possible explosion of hydrogen.

In the commencement of the process, or at any time if desired, fresh sodium hydroxide may be employed for the elimination of the arsenic. It is preferable, however, to operate so that the In other words, aiterthe arsenic content been built upto a high point in the,dr0ss, dross is oil from the metal bath then fresh caustic soda containing prefernoarsenicisshoveled ontothebathand iinal proportion of arsenic in the molten moved. At this point, then, following the prooedure mentioned above, iresh sodium hydroxide is shoveled on and the process so continued.

Arsenic in the bullion can be readily reduced to traces by this method and final slags with very little antimony and lead can be made.

The following are examples of the process as we now prefer to practise it. The invention is not to be consideredas limited thereby except as indicated in the appended claims.

Exmul 7,740 lbs. of metal assaying 36% As, 6.16% Sb and-the balance substantially lead, were placed in a kettle and melted down and the temperature raised to about 1000' F. To this was added lbs. of fresh sodium hydroxide which is equivalent to about 2.8 lbs. of hydroxide per pound of arsenic. The impeller wasplaced in the molten metal bath and the steel-plate cover device placed on the kettle to exclude the air. The stirrer provided with a. 6% inch impellerwasrunataspeedof l'fllilRRMfor two hours with the metal batli' between lo00 and 1100' F. A sample of the metal taken after 1 hour of stirring showed the arsenic to be reduced to 197%. while at the end of the second hour the arsenic was reduced to 133%.

At the end of two hours the liquid dross was removed, weighed and The following metal balance shows th results obtained:

Analysis, per cent Material Amount As Sb Pb s ii ittisiasan um I) Final metal.

. 7,740 .36 Bel. 114 22. 10 51 7,700 .03 Bill.

The rate ofremoval of the first hour and secand hour based on metal assays is as iollows:

Rate of Pounds removal oi arssni arsenic Per removed 0, pounds per cent Metal hour per ton Per hour The assays of the caustic dross removed show the very excellent removal of arsenic with a very small removal of antimony and lead. The actual Exmrtr: 2

The following is a two-step process for reducing arsenic to a trace:

Step 1.--To 7,380 lbs. of lead containing .88% arsenic, .73% antimony, balance substantially lead and contained in a kettle as in Example 1 I with air excluded, was added 185 lbs. fresh sodium hydroxide during the first fifteen minutes of stirring. Stirring was carried on at the rate of 1700 R. P.-M. with a 6 /2 inch impeller. The temperature at the start of the stirring period was 850 F. and this was gradually raised to 1100 F. After 5 hours of stirring, the metal assayed .13% As and after 6 hours, it assayed .10% As. The slag was removed at the end of 6 hours. It was liquid, weighed 222 lbs. and assayed 29.13% As, .46% Sb and a trace of lead.

Step 2.To the metal remaining in the kettle after 6 hours stirring was added 100 lbs. of fresh sodium hydroxide and after stirring 50 minutes,

the metal contained only a trace of arsenic. The

slag removed at that time weighed 115 lbs. and assayed 9.58% arsenic, .91% antimony and .20% lead. This slag may be charged to a new batch of lead, containing arsenic, to. be treated or otherwise disposed of.

The following is a tabulation of the results The following example shows the use of (a) fresh sodium hydroxide on a lead-arsenic bath to produce a dross having a low arsenic content, and (b) the use of this dross on a second bath .to produce a second dross having a higher arsenic content. This second metal bath is then treated with fresh hydroxide to produce a dross having alow metal content and then the process is repeated using this low arsenic dross on a new metal bath as in step b. To 54,888 parts of metal assaying 241% of arsenic and .65% of antimony and the remainder lead, there was added 975 parts of fresh sodium hydroxide to produce a dross containing 8.43% arsenic and 2.42% antimony. The final metal carried less than .005% arsenic and .60% antimony. These figures are set forth in Table a below and it will be noted that the original metal is designated as "(a) Head metal": the sodium hydroxide is designated as (11) NaOH (hydroxide) th dross as 9(a) Dross," and the final metal as "(11) Final metal."

Table a Analysis, per cent Parts As 81) Pb a Head metal 888 .241 .65 B gag NaOH (hydroxide) '075 I (a Dross 1,385 8.43 2.42 (a) Fmal metal 54.445 Legfithan .60 Balance In carrying out step b above, (a) dross was then put on a fresh metal bath and the process continued, to produce a dross which was then removed; whereupon fresh sodium hydroxide was added to produce a dross and final metal, and this dross was then available for use with a head metal such as that employed at the start of step b.

The 1,385 parts of (a) dross above was accordingly added to 54,940 parts of lead containing 577% arsenic and .6% antimony and after stirring with air excluded, there were produced 1,365 parts of dross assaying 23.2% arsenic, .75%

antimony, .64% lead. The 54,940 parts of leadof fresh sodium hydroxide reduced the arsenic content of the said metal to less than .005%. In Table b this sodium hydroxide is designated as "Fresh NaOH; the dross is designated as (c) Dross and the metal obtained is designated as (b) Final metal." (17) Dross may then be treated to recover the arsenic or thrown away and (c) Dross may be put on a fresh bath of metal containing relatively large quantities of arsenic to be removed, such as (a) or (b) Head metal above, and the process then continued to remove this arsenic. When the arsenic content of this dross has risen sufllciently, it is then re moved and fresh sodium hydroxide placed on the metal to reduce the arsenic content to somewhere in the neighborhood of .005%. In other words, the process is a cyclic one, as indicated in the statement at the beginning of this example.

t tlfesults of this run are shown in the following Table b Analysis, per cent Parts As Sb Pb 577 Balance 1, 385 8. 43 2. 42 9) Dross-" 1,365 23.2 .75 .64 real: NaOH (hydroxide). 425 (c) Dross 795 16.3 1.3 .50 (0) Final metal 54,810 Legsosthan .65 Balance Instead of sodium hydroxide mentioned in the above examples, we may also employ other caustic alkali such as potassium hydroxide, but we prefer sodium hydroxide because of its lower cost.

What we claim is:

1. A process for treating impure lead containing arsenic and antimony which comprises melting the impure lead, agitating it with caustic alkali substantially in the absence of air in con- 4- aasmss tact with am? part of the bath and forming a dross containing arsenic but with no substantial amount 01' antimony.

2. A process for treating impure lead containing arsenic and antimony which comprises stirring the impure lead in molten condition with caustic alkali substantially in the absence oi air in contact with any part oi the bath while maintaining a temperature 0! the order 01 1000-1100 I". and forming a dross containing arsenic but 10 with no substantial amount of antimony,

3. A process for treating impure lead containing arsenic and antimony which comprises agitating the impure molten lead with sodium hydroxide while maintaining the temperature at is antimony.

about 1000-1100 F. substantially in the absence of air in contact with any part at the bath and forming a dross containing arsenic but with no substantial amount of antimony.

4. A process for treating impure lead contain-- 5. A process for treating impure lead contain- 7 in: arsenic and antimony which comprises meltin: the impure lead in an air-tight kettle and stirrins it substantially in the absence of air in contact with any part of the bath while mainmm; a temperature at about 1000-1100' F. with sodium hydroxide, and forming a dross containin: arsenic but with only a small amount of NEVIN R. BIERLY. 

